Signal Processing in GPSGPS modules have to evaluate weak antenna signals from at
least four satellites, in order to determine a correct
three-dimensional position. A time signal is also often emitted in
addition to longitude, latitude and height. This time signal is
synchronized with UTC (Universal Time Coordinated). From the position
determined and the exact time, additional physical variables, such as
speed and acceleration can also be calculated. The GPS module issues
information on the constellation, satellite health, and the number of
visible satellites etc.
The signals received (1575.42 MHz) are pre-amplified and
transformed to a lower intermediate frequency. The reference
oscillator provides the necessary carrier wave for frequency
conversion, along with the necessary clock frequency for the processor
and correlator. The analogue intermediate frequency is converted into
a digital signal by means of a 2-bit ADC.
Signal transit time from the satellites to the GPS receiver is
ascertained by correlating PRN pulse sequences. The satellite PRN
sequence must be used to determine this time, otherwise there is no
correlation maximum. Data is recovered by mixing it with the correct
PRN sequence. At the same time, the useful signal is amplified above
the interference level [xxv]. Up to 16 satellite signals are processed
simultaneously. The control and generation of PRN sequences and the
recovery of data is carried out by a signal processor. Calculating and
saving the position, including the variables derived from this, is
carried out by a processor with a memory facility.